Detectron2 add custom class

[unrue]

Dear Detectron2 users,

I'm using the following Colab in order to add a custom class:

https://detectron2.readthedocs.io/en/latest/tutorials/getting_started.html

It works well, but my question is: the example starts from a pretrained coco net, so why the inference detect only ballons and not persons in the examples at the end? Person class are preset in coco dataset. The trained net seem to "forget" the coco classes and detect only new added from my self. How can I have a net that detect all default coco classes and new classes added? Thanks.